1. Field of the Invention
The present invention relates to an inkjet printing apparatus for printing an image by using thermal energy and an inkjet printing method thereof. In particular, the present invention relates to a control method of a print head in an inkjet printing apparatus for alleviating an image defect caused by a temperature distribution within the print head.
2. Description of the Related Art
In the inkjet printing apparatus, thermal energy is provided to a plurality of print elements arranged in the print head according to image data to eject ink from the individual print elements, thus printing an image on a print medium. In such an inkjet printing apparatus, an ink temperature in the print element is influenced by ejection frequency of the print element or print elements in the surroundings thereof, and as the ink temperature is higher, an ejection amount of the ink also becomes the larger. Therefore there are some cases where even within the same print head, the ejection amount varies depending on irregularities of the ejection frequency or the ejection amount changes in accordance with an elapse time from a print start, inviting the density unevenness in the image on the print medium.
For example, Japanese Patent Laid-Open No. H05-031905(1993) discloses an ejection amount control method (PWM control) for solving this problem. According to the PWM control, there is disclosed the method in which a pulse width of a voltage pulse applied to each of the print elements is adjusted in accordance with a temperature of a chip in which a plurality of print elements are arranged, and even if a temperature change occurs in the chip, the ejection amount can be kept constant. In addition, Japanese Patent Laid-Open No. H06-336022(1994) discloses the method in which a sub heater, which heats a print head to a temperature at which a stable ejection is ensured, is controlled in response to a detection temperature of a temperature sensor arranged near the print element.
According to Japanese Patent Laid-Open No. H05-031905(1993) or Japanese Patent Laid-Open No. H06-336022(1994), it is required for a temperature distribution of the plurality of the print elements to be detected as accurately as possible. As the detection error is large as compared to an actual temperature distribution, the ejection amount control can not be normally performed, so that the density unevenness can not be alleviated or the density unevenness is rather worsened. Therefore in recent years, there is provided an inkjet printing apparatus in which, with the aim of accuracy improvement on the temperature detection, a plurality of temperature sensors are arranged on a single chip to determine the detection temperatures in a comprehensive manner, thus performing the drive control to print elements at ejection. For example, Japanese Patent Laid-Open No. 2000-334958 discloses the method for performing PWM control based upon an average value of a plurality of detection temperatures obtained from a plurality of temperature sensors. In addition, Japanese Patent Laid-Open No. H10-100409(1998) discloses the method for weighting each of the detection temperatures corresponding to a position of the temperature sensor on a chip to determine a representative temperature for the drive control.
However, in some cases the method for finding the representative temperature for the drive control according to Japanese Patent Laid-Open No. 2000-334958 or Japanese Patent Laid-Open No. H10-100409(1998) does not work appropriately in a case of a full line type of inkjet printing apparatus.
The full line type of inkjet printing apparatus uses a print head in which a plurality of chips are arranged to the extent corresponding to a width of the print medium, each chip having a plurality of print elements arranged thereon. Ink is ejected on the print medium moving in a direction crossing the arrangement direction of the print elements from each print element, thus printing an image on the print medium. In such a full line type of inkjet printing apparatus, printing can be performed on print media having various sizes as long as the image is equal to or less than the arrangement width of the chips, but in this case, only the limited chips or the print elements in the limited region are used for printing, and a temperature gradient within the print head becomes large. Also in this situation, the temperature detection method disclosed in Japanese Patent Laid-Open No. 2000-334958 or Japanese Patent Laid-Open No. H10-100409(1998) can be adopted, but since a detection temperature in a region not used in printing is also used for determining the representative temperature, there occurs a possibility that the temperature in the region used in printing can not be accurately detected. That is, in the full line type of inkjet printing apparatus, even if the method disclosed in Japanese Patent Laid-Open No. 2000-334958 or Japanese Patent Laid-Open No. H10-100409(1998) is adopted, there occurs concern that the density unevenness may not be reduced or may be rather worsened depending upon image data.